The Global Positioning System (GPS) is a versatile, generally available worldwide navigational system based on the reception of signals from a network of satellites orbiting the globe. GPS is widely used as an external source for geo-location in 2D and 3D Local Positioning Systems (LPS), e.g., automobile navigation and unmanned aerial vehicles (UAV). The growing reliance upon a single source of positioning information introduces a significant vulnerability. Since GPS is a weak signal, it is vulnerable to jamming. Furthermore, the multitude of satellites forming GPS was intended to provide accurate signals and not necessarily support a degree of fault-tolerance, and so GPS is vulnerable to spoofing (i.e., the purposeful sending of inaccurate data to the receiver). The European Union (Galileo), Russia (GLONASS), China (BeiDou) and other countries have deployed and are in the process of deploying more satellites in order to improve the accuracy and reliability of Global Navigation Satellite Systems (GNSSs). Nevertheless, it has been pointed out that the most severe limitation of GNSS performance will still remain; the accuracy of positioning deteriorates very rapidly when the user receiver loses direct view of the satellites, which typically occurs indoors, or in severely obstructed urban environments, steep terrain and in deep open-cut mines. As a result, LPS are often used in either a complementary or alternative fashion to the satellite systems, especially in areas where GPS signals are sufficiently degraded. Although Long-range LPS, e.g., Decca Navigator System and LORAN (Long Range Navigation), have been used for navigation of ships and aircraft, nowadays, a LPS typically refers to a system with limited range used for outdoor and/or indoor applications.
Positioning systems that consists of a network of three or more signaling beacons have been used for navigation and surveying by providing location information within the coverage area. The reliability and accuracy of such a positioning system fundamentally depends on two factors; first, its timeliness in broadcasting signals, i.e., whether or not the signals are transmitted at the same time or as close to the same time as possible, and second, the knowledge of its geometry, i.e., locations and distances of its beacons. The more accurate the time at each beacon and the higher the precision across the network, the more accurate the estimated position at the receivers. Similarly, the more accurate the geometry and knowledge of the location and distances of the beacons from each other, the more accurate the estimated position at the receivers will be.
Distributed LPSs (DLPS) either synchronize to a common external source like GPS or establish their time synchrony internally. GPS satellites operate on very high precision atomic clocks that “tick” with an accuracy of one nanosecond (providing position accuracy within 5 to 10 meters) and are synchronized to the Coordinated Universal Time (UTC), which is the primary time standard by which the world regulates clocks and time. The atomic clocks on these satellites are very accurate (e.g., drift rate 10-13, or less) and very expensive, thus, cost prohibitive for most applications. LPSs on the other hand operate with lower quality clocks, i.e., have a higher drift rate, that are more affordable. These clocks, however, need to be periodically resynchronized to account for their inherent drift and far more frequently than the atomic clocks.
In DLPSs that use master-slave scheme to internally establish their time synchrony, typically, a particular beacon is designated as the master and other beacons synchronize to it, e.g., the Locata system (Locata Corporation, Canberra, Australian Capital Territory, Australia). Due to its centralized nature, a network with master-slave scheme results in a single point of failure. Another drawback of existing DLPSs is their lack of addressing various fault manifestations, in particular, communication link failures. Firstly, due to the high reliability of modern processors, communication-related failures like receiver overruns (run out of buffers), unrecognized packets (synchronization errors), and CRC errors (data reception problems) in all sorts of wireless networks are increasingly dominating process failures. Secondly, such link failures are typically transient and mobile, in the sense that they typically affect different messages to from different processes over time.
The geo-location and time-synchrony problems have a lot in common. Geo-location requires a distributed system of at least four beacons to estimate the location in 3-dimentions. The fourth beacon is necessary to account for discrepancies in value of time readings, ((x, y, z), t), which is primarily due to low quality of the receiver's local oscillator. Similarly, the time-synchrony problem requires a minimum of four nodes (beacons) to tolerate a malicious faulty behavior. The capability of a distributed network of beacons to autonomously self-synchronize and, subsequently, provide reliable signaling for proper geo-location, at the receivers and independent of GPS, is essential in reducing total reliance on an external source like GPS. The internal timing of an independently self-synchronizing network can readily be realigned to GPS time when it becomes available. Similarly, prior knowledge of the exact locations of the beacons and their network geometry is not necessary; an autonomous distributed fault-tolerant local positioning system should be able to first determine its own geometry and then realign it to the world map when GPS data is available.
Thus, it may be desirable to provide a highly reliable, GPS-independent, fault-tolerant, redundant system for geo-location of UAVs, such as those UAVs used in NASA's Unmanned Aircraft Systems (UAS) Integration in the National Airspace System (NAS) Project and Langley Research Center's CERTAIN (City Environment for Range Testing of Autonomous Integrated Navigation). In addition, it may be desirable to provide a DLPS suitable for high-dynamic systems by accommodating capabilities for UAVs to maneuver at high speed, and in dynamic and mobile environments.